Etching processes such as chemical etching and electrochemical etching can be used to carve into the exteriors of ferrous alloy or other metal objects which are not easily engraved by other means. Etching, sometimes referred to as milling, uses corrosive chemicals or electric current to engrave patterns into a substrate. Generally, a masking material or coating is applied to the substrate and removed from areas that are to be etched. The substrate may then be exposed to an etchant to form a desired pattern in the substrate.
In chemical etching processes, the coatings are resistant to the corrosive properties of a chemical etchant. For electrochemical etching, or EChE, the coating is electrically non-conductive, and is thus resistant to ablation of the substrate surface by an electric current. For example, in an EChE process, an electric circuit is established with a suitable cathode fixed at a desired distance from the substrate, which acts as the anode. An electrolyte compatible with both anode and cathode materials is introduced between the cathode and anode, and then current is passed through the circuit. Metal ions from the exposed portions of the substrate are dissolved or dislocated into the electrolyte at a rate proportional to the current applied.
In all etching methods, the coating needs to be removed from the substrate in the desired pattern so that exposure to the etchant may remove a portion of the substrate. For non-uniformly shaped objects, such as an exterior of a three dimensional object, this may not be a straight-forward process. Furthermore, removing the coating in a pattern has generally not been possible on interior surfaces of certain objects such as hollow objects due to the lack of appropriate tools and processes. That is, no technology for scribing and etching the internal surfaces of hollow, three-dimensional objects or shapes composed of ferrous alloy or other substrate is readily available.